Demisters or mist eliminators are devices used for removing entrained liquid droplets from a gas stream and are often necessary to be used where any liquid entrainment in vapor stream is not acceptable, for example—compressor suction lines. As its name implies, a demister is used for removal of the mist from the gaseous phase. Demisters, which are often fitted just below the top vapor outlet of a vapor liquid separator, function by coalescing smaller liquid droplets into larger droplets by obstructing their path. Obstruction of path causes increased collisions among the liquid droplets and most of the droplets stick together and form bigger droplets which are too heavy to rise with the gas stream. The bigger liquid drops then fall into the pool of liquid below. The gas component of the stream is not affected by the obstruction in the path and escapes through the vapor outlet at the top of the demister.
The obstruction in the path of liquid droplets can be achieved by a variety of geometries. There are two general types of mist eliminators—mesh and vane. Demisters may be mesh type coalescers, vane packs and these may both assist empty drums by increasing the gas and liquid separation efficiency. One widely applicable type of mist eliminator is the Crinkled Wire Mesh Screen (CWMS) which is made of metal or plastic wire with typical diameter of 0.1 to 0.5 mm, loosely knitted in a form resembling a cylindrical net. This tube is flattened to form a two-layer strip, which is then crimped in a diagonal pattern with ridges; when these strips are laid together, the ridges slant in alternate directions, forming an open structure through which gas flows freely. Such mesh pads can efficiently capture mist droplets as small as 5 microns.
The second main type of demister, the vane or chevron type, comprises of closely spaced corrugated plates that force mist-laden gas to follow serpentine paths. These devices are generally not efficient for mist droplets smaller than about 20 microns, but they are more fouling resistant and sturdier than mesh pads, and impose a lesser pressure drop. Vane arrays can be mounted horizontally or vertically. They are preferred in applications involving high vapor velocities, low available pressure drop, viscous or foaming liquids, lodging or caking of solids, slugs of liquid, or violent upsets. They are sometimes used in combination with mesh pads for optimum performance in certain situations.
U.S. Pat. No. 4,072,478 (Regehr) discloses a vane type demister in which a number of generally V-shaped vanes are arranged between partition walls to define rectangular channels through which the gas stream flows. Each vane has a chevron shaped directing groove with flat run-off walls between the ends of the grooves and the partitioning walls.
Another demister is described in U.S. Pat. No. 4,204,847 (Ko) with chevron shaped baffles or vanes disposed in the gas stream with the upstream and downstream parts of each gas flow passage between the baffles oriented in different planes. Mist trapping flanges project into the upstream and downstream branches of each of the flow passages.
U.S. Pat. No. 5,203,894 (Chowaniec) describes a mist eliminator pack made up of non-parallel sheets with integrally formed offset tabs in the first edges of the sheets which are secured to the adjacent sheets to form an integral pack with uniform flow paths between the sheets.
The mist eliminator described in U.S. Pat. No. 5,464,459 (VanBuskirk) has a number of generally chevron-shaped impingement baffles with short entry and outlet sections and lower and upper sections arranged at different angular inclinations. The lower end of the entry section may have a toothed configuration to promote drainage of accumulated liquid.
Traditional vane-type mist eliminators are designed for moderate fouling services and span the entire cross sectional area of the column or tower in which they are located. Over time with continuous operation, the mist eliminators can begin to foul and the pressure drop through the device may increase gradually up to the design limit. Unit capacity may be reduced to prevent excessive pressure drop and equipment failure but eventually, the fouling becomes excessive and results in poor liquid droplet removal performance from the vapor phase to the extent that the mist eliminator requires replacement. Frequent unit outages for inspection and maintenance results in significant cost and loss of revenue.